1. Field of the Invention
The present invention relates to a leadframe, and more particularly to a leadframe for a resin-sealed semiconductor device.
2. Description of the Related Art
As shown in FIG. 9, a conventional type of leadframe 1, 1' is typically constructed in a single layer structure using a single material such as a copper(Cu)-based material or an iron-nickel(Fe--Ni) alloy (generally known as Alloy 42). In the illustrated construction, a chip 4 containing a semiconductor integrated circuit is mounted on the upper principal surface of a stage 1 of the leadframe with a conductive adhesive, such as silver(Ag) paste, intervening therebetween for bonding one to the other; after a pad 7 forming a terminal on the chip 4 is connected to a lead 1' of the leadframe (the tip of the lead 1' is plated with silver(Ag) or the like) using an appropriate lead wire such as gold wire, an appropriate sealing resin 2 is molded to encapsulate the stage 1 of the leadframe, the chip 4, and a portion of each lead 1'. A resin-sealed semiconductor device is thus formed.
In a semiconductor device using a leadframe of the above construction, adhesion between the chip and the leadframe and between the leadframe and the sealing resin becomes a problem; while the bonding strength at the interface between the leadframe and the chip has been improved by the improvement of adhesives, the bonding strength at the interface between the resin 2 and the leadframe, and in particular stage 1 thereof, is not yet sufficient, and an improvement in the bonding strength is required.
Specifically, if the bonding strength at the interface between stage 1 and resin 2 is weak, imperfections such as voids, cracks, and gaps may be initially contained in the interface, or such voids, cracks, gaps, etc. may be further developed or expanded when stress is applied to the semiconductor device.
In particular, if such voids, cracks, gaps, etc. are formed in the interface, there is a possibility that water or moisture initially contained in resin 2 and, furthermore, external water or moisture subsequently absorbed into resin 2, will build up in the voids, cracks, gaps, etc. The resulting problem is that when mounting the semiconductor device on a circuit board, the high temperature of soldering (e.g., 215.degree. C. to 260.degree. C.) used in the process will cause the water or moisture to expand in volume, thereby expanding the voids, etc. and thus forming cracks in the semiconductor device.
To overcome the above problem, the prior art has employed a method that involves providing through holes in stage 1 of the leadframe. This method, however, results in a significant reduction in the bonding effect since the conductive adhesive, such as silver paste or solder, enters the through holes when the chip 4 is mounted on the stage 1. The method also involves a reduction in the bonding strength between stage 1 and the chip 4 because of the reduced bonding area between them.